1. Field of the Invention
The present invention relates to a multi-layer solid golf ball comprising at least a core, an inner layer cover, and an outer layer cover. More particularly, it relates to a golf ball in which convex ribs are formed on the inner surface of the outer layer cover such that the convex ribs intrude into the inner layer cover.
2. Related Art
Various techniques have been studied and proposed for increasing travel distance of a golf ball and providing a player with an excellent feel upon hitting the golf ball (hereinafter may be called "hit feel"). Especially in a golf ball having a solid core and a cover, the hardness and size (diameter and thickness) of the core and the hardness and size of the cover are adjusted for such purposes.
For example, U.S. Pat. No. 5,439,227 discloses a three-piece golf ball which has a solid core, an inner layer cover, and an outer layer cover and in which the outer layer cover is made harder than the inner layer cover. Also, U.S. Pat. No. 5,490,674 discloses a three-piece golf ball which has inner and outer solid cores covered with a cover and in which the inner solid core is made harder than the outer solid core.
In the above-described golf balls, the boundary surface of each layer is generally a smooth spherical surface having neither projections nor depressions. However, U.S. Pat. Nos. 2,376,085 and 5,692,973 disclose a golf ball which has on its solid core projections for preventing eccentricity of the solid core, which eccentricity could otherwise arise when a cover is formed around the core through injection molding.
The projections on the solid core of the above-described golf ball are designed to substitute support pins used in an injection molding process, and the effect obtained by the shape of the support-pin-shaped projections is not utilized to improve the performance of the golf ball. In other words, the inventions of U.S. Pat. Nos. 2,376,085 and 5,692,973 relate to a technique for preventing eccentricity of the solid core and preventing mixture of a different material into the cover. According to the technique, by employment of the same material as used for the cover, projections are formed on the core surface such that the cover has a uniform thickness, and the projections and the cover are thus united. As described above, the projections are not designed to improve the performance of the golf ball.
Also, Japanese Patent Application Laid-Open (kokai) No. 9-285565 discloses a two-piece golf ball which has projections and depressions between a solid core and a cover, between two adjacent layers of a multi-layer solid core, or between two adjacent layers of a multi-layer cover. The two-piece golf ball provides a player with different hit feels, depending on the direction of an external force acting on the golf ball during hitting.
The two-piece golf ball has improved in terms of hit feel provided to a player. However, the travel performance and durability are not satisfactory, and there is room for further improvement.